The invention relates generally to the field of wireless communication systems and more particularly to frequency reuse schemes utilizing base station antennas with notched patterns.
A communication system generally includes three components: transmitter, channel, and receiver. The transmitter modulates the signal over the channel. The receiver demodulates the received signal to produce an estimate of the original message. The channel provides a connection between the transmitter and the receiver.
Two types of two-way communication channels exist, namely, point-to point channels and point-to-multipoint channels. Examples of point-to-point channels include wires (e.g., local telephone), microwave links, and optical fibers. Point-to-multipoint channels enable multiple receivers to be reached simultaneously from a single transmitter (e.g. cellular radio telephone communication systems and Local Multipoint Distribution Systems (LMDS)). For broad geographical coverage, a plurality of geographically dispersed transmitters may be employed, each one""s coverage area being known as a cell. Directional antennas may be employed at the transmitters, each one covering a portion of a cell known as a sector.
One factor that limits the capacity of a point-to-multipoint radio system is interference from transmitters using the same frequency in different cells, thus restricting the allocation of frequencies However, there are some known frequency reuse schemes which improve the available capacity.
Typically the present state of the art limits reuse of frequencies to N=4 (i.e. each sector operates in a frequency chosen from a predetermined group of four frequencies) for high order modulation schemes. Although reuse schemes have been proposed based on N=2 or N=1, these schemes are in many cases dubious in performance, particularly for high level modulation schemes such as 16 Quadrature Amplitude Modulation (QAM) or 64 QAM, since they typically fail in deployment.
Cellular frequency reuse schemes have been designed and employed with omnidirectional antennas at the terminal equipment and with hexagonal cells. However, in conventional LMDS systems, highly directional antennas are used at the remote terminal equipment with quadratic orientation of base station antennas (i.e. four-sided cells).
FIG. 1A depicts a cell in a conventional LMDS system. A base station 10 is nominally at the center of cell 20. Base station 10 is equipped with four antennas (not shown), each having a nominal 90-degree coverage pattern. The four antennas are aimed 90 degrees apart, thus dividing the cell 20 into four sectors 30. Each of the four antennas operates on a different frequency, each frequency represented by a different orientation of hatching in a sector 30.
FIG. 1B illustrates an LMDS frequency reuse scheme employing a plurality of base stations arrayed on the landscape so as to cover a large area with overlapping cells. The same four frequencies (represented by the four orientations of hatching) are reused a number of times. Some cells use the four frequencies in different positions than the representative cell of FIG. 1A.
An analysis of the scheme illustrated in FIG. 1B shows that with the highly directional antennas at the remote terminal equipment, interference is localized to very narrow zones along the lines connecting multiple base stations. For example, communications to a remote unit located in sector A would be interfered with by base station transmissions in sector B, but not from transmissions from sectors C, D, or E even though sectors C, D, and E operate on the same frequency as sectors A and B. Thus, the interference zones reflect the antenna patterns of the remote terminal equipment.
However, there still exists a need for a better reuse system in LMDS.
There is a need for systems which use base-station antennas that are omnidirectional or xe2x80x9cpseudo omnidirectionalxe2x80x9d (i.e., exhibiting radiation over nearly all, but not entirely all, of 360 degrees.
Accordingly it is an advantage of the present invention to provide an interference-reducing frequency reuse scheme for use with LMDS.
It is another advantage of the present invention to provide a reuse scheme in which Local Multipoint Distribution Systems (LMDS) base stations use omnidirectional or pseudo-omnidirectional base-station antennas with reduced interference.
These and other advantages of the invention will become apparent to those skilled in the art from the following description thereof.
In accordance with the teachings of the present invention, these and other advantages may be accomplished by the present systems and methods of frequency reuse schemes by employing notched antenna patterns.
In an exemplary embodiment of the invention, the system includes a frequency reuse scheme employing omnidirectional or pseudo-omnidirectional antenna patterns whereby the sector antenna pattern is designed to have directional notches in order to control interference to the surrounding cells.
In another embodiment the system includes a frequency reuse scheme whereby the angular locations of the notches in the antenna pattern are substantially matched to the positions and antenna patterns of the surrounding cells sharing that frequency.
In another embodiment the system includes a frequency reuse scheme whereby orthogonal polarization may be made available for co-channel in-cell repeaters.
In still another embodiment the system includes a frequency reuse scheme whereby the orthogonal polarization may be used for radio backhaul to a central base station from its surrounding base stations (either co-channel, adjacent channel or spread over multiple channels).
In another embodiment of the invention, the system includes a frequency reuse scheme whereby all channels may be used in all cells of a network.
Still another embodiment of the invention includes a frequency reuse scheme whereby all channels may be used in all cells of a network and reused to provide in cell repeaters using either copolar or cross polar polarization (dependent on location of the repeater).
In another embodiment of the invention, the system includes a frequency reuse scheme whereby intercell communication (e.g. backhaul) may be provided using a subset of all channels, the subset being determined by the frequency space patterns of the relevant cell pair.
The invention will next be described in connection with certain exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.